The present invention relates to fluid pressure cylinders used to move objects in various production or processing facilities.
In a typical fluid pressure cylinder, a cushioning structure is used to decelerate the associated piston at an end of its stroke. In many of the conventional fluid pressure cylinders, the outer diameter of an end cover is larger than the outer diameter of a cylinder tube. A cushion valve extends to the upper side of the end cover, for example, and a piping port opens on a side of the end cover. To meet a demand for a reduction in the size of a fluid pressure cylinder and for savings in constituent materials, it was conceived to arrange a fluid pressure cylinder as shown in FIG. 3. That is, the diameter of an end cover 12 is set equal to the inner diameter of a cylinder tube 11, and the end cover 12 is fitted to the inner surface of the cylinder tube 11.
In the prior art shown in FIG. 3, the cylinder tube 11 has a large-diameter portion 11a defined with a step portion formed on the inner surface of an end portion thereof. The end cover 12 has a large-diameter portion 12a defined with a step portion formed on the outer surface of an end portion thereof. The end cover 12 is inserted into the end portion of the cylinder tube 11, and the large-diameter portion 12a of the end cover 12 is fitted to the large-diameter portion 11a of the cylinder tube 11. The position in the axial direction of the end cover 12 is determined by engagement between the step portions of the end cover 12 and the cylinder tube 11. The large-diameter portion 11a of the cylinder tube 11 is provided with an annular groove, and a retaining ring 13 is fitted in the annular groove to prevent the end cover 12 from falling out of the cylinder tube 11. A piston 15 is slidably fitted to the inner surface of the cylinder tube 11. A piston rod 16 is connected to the piston 15. A cushion ring 18 is formed on a side of the piston 15 closer to the piston rod 16, and a cushion plunger 17 is formed on the other side of the piston 15. The end cover 12 has a center bore 21. The center bore 21 has a small-diameter portion 21a, an intermediate-diameter portion 21b, and a large-diameter portion 21c in order from the outside in the axial direction (i.e. from the left side in FIG. 3). The small-diameter portion 21a is pierced by the piston rod 16 with a seal 22 provided therebetween. The large-diameter portion 21c is fitted with a cushion packing 37. The piston 15 has a packing 19 fitted in an annular groove on the outer periphery thereof. The packing 19 hermetically seals the area of sliding contact between the piston 15 and the cylinder tube 11.
A piping port 23 is formed in the cylinder tube 11 at a position near the end thereof. The piping port 23 is communicated with the intermediate-diameter portion 21b of the center bore 21 in the end cover 12 through a first communicating passage 24. The cylinder tube 11 is provided with a cushion valve control hole 25 axially adjacent to the piping port 23. The end cover 12 is provided with a cushion valve inserting hole 26 in a coaxial relationship to the cushion valve control hole 25. The cushion valve inserting hole 26 has a large-diameter portion, a threaded-bore portion, a valve chamber 28, and a valve seat portion in order from the outside in the radial direction of the end cover 12. A cushion valve (needle valve) 27 is inserted into the cushion valve inserting hole 26. The valve chamber 28 is communicated with a rod-side chamber 30 in the cylinder tube 11 through a second communicating passage 29. The valve chamber 28 is also communicated with the intermediate-diameter portion 21b of the center bore 21 in the end cover 12 through a valve seat. The cushion valve 27 has a large-diameter portion, an external thread portion, and a needle portion in order from the outside in the radial direction of the end cover 12. The external thread portion is engaged with the threaded-bore portion of the cushion valve inserting hole 26. The space between the cushion valve 27 and the cushion valve inserting hole 26 is hermetically sealed with an O-ring fitted in an annular groove on the large-diameter portion. By rotating the cushion valve 27, the gap between the needle portion and the valve seat is varied to control the flow rate of a fluid flowing through a bypass flow path (including the second communicating passage 29) that provides communication between the intermediate-diameter portion 21b of the center bore 21 and the rod-side chamber 30.
The outer peripheral surface of the end cover 12 is provided with a first annular gasket groove 33, a second annular gasket groove 34, and a third annular gasket groove 35 such that the three gasket grooves 33, 34 and 35 lie in three planes, respectively, which are perpendicular to the center line X--X of the cylinder tube 11. The first annular gasket groove 33 is located axially outward of the first communicating passage 24. The second annular gasket groove 34 is located between the first communicating passage 24 and the cushion valve inserting hole 26 with respect to the axial direction. The third annular gasket groove 35 is located axially inward of the cushion valve inserting hole 26. The first, second and third annular gasket grooves 33, 34 and 35 are fitted with a first annular gasket 33a, a second annular gasket 34a and a third annular gasket 35a,respectively. It should be noted that illustration of the reference characters of the three gaskets is omitted in FIG. 3. The first annular gasket 33a hermetically seals the joint between the cylinder tube 11 and the end cover 12 to prevent communication between the atmosphere on the one hand and the piping port 23 and the first communicating passage 24 on the other. The second annular gasket 34a hermetically seals the joint between the cylinder tube 11 and the end cover 12 to prevent communication between the piping port 23 and the first communicating passage 24 on the one hand and the cushion valve control hole 25 and the cushion valve inserting hole 26 on the other. The third annular gasket 35a hermetically seals the joint between the cylinder tube 11 and the end cover 12 to prevent communication between the cushion valve control hole 25 and the cushion valve inserting hole 26 on the one hand and the rod-side chamber 30 on the other.